1. Field of the Invention
The present invention relates in general to a driving system for a discharge tube. In particular, the present invention relates to a multi-lamp driving system for a liquid crystal display backlight.
2. Description of the Related Art
A discharge tube, particularly a cold cathode fluorescent lamp, CCFL), is used as the light source for LCD backlight systems. The CCFL is typically driven by an inverter circuit, which provides DC current to a lamp and includes a feedback and control loop for stabilizing lamp currents. A large LCD panel usually requires two or more fluorescent lamps to provide sufficient backlighting.
FIG. 1 (Prior Art) is a multi-lamp driving system in a conventional device. The system includes a driving circuit 10 and a transformer T1 for driving two fluorescent lamps LP1 and LP2, and has one feedback and control circuit 30. The impedance difference of the lamps seriously influences the currents I1 and I2 though the lamps and therefore unbalances the distribution of the currents. However, the circuit in the FIG. 1 only controls the total current of the lamps LP1 and LP2 and fails to balance the current through each of the lamps. The imbalance not only affects the illumination uniformity of the LCD panel due to insufficient luminance of those lamps having too small currents, but also shortens the life of individual lights and the backlight system due to overheat of those lamps having too large current.
In order to overcome the above disadvantage, some schemes using several driving circuits and transformers to drive a plurality of lamps and provide several feedback and control circuits for current regulation are proposed. As shown in FIG. 2, driving circuits 10,20 and transformers T1 and T2 are provided to drive lamps LP1 and LP2. In addition, the lamps LP1 and LP2 are connected to feedback and control circuits 30 and 40. This scheme balances the currents of lamps I1 and I2, but requires more components and increases fabrication costs and product size.
Therefore, it is necessary to provide a circuit balance technique for multi-lamp driving systems to solve the above problems.
An object of the present invention is to provide a multi-lamp driving system, which directly controls the current balance among lamps.
Another object of the present invention is to provide a low-cost, small-scale multi-lamp driving system.
The multi-lamp driving system of the present invention is used to drive a lamp set having a first lamp and a second lamp. It comprises a driving circuit for converting a DC (Direct Current) signal to an AC (Alternating Current) signal, a transformer having a primary side coupled to the driving circuit and a secondary side to output AC signal, and a current balance circuit coupled to the lamp set to balance the currents flowing through the first lamp and the second lamp. The current balance circuit comprises a magnetic core, a first winding connected to the first lamp and a second winding connected to the second lamp. The first winding and the second winding are wound on the magnetic core and have the same number of turns. The current balance circuit is coupled to a high voltage terminal or low voltage terminal in the lamp set based on various applications.
Optionally, the multi-lamp driving system of the present invention further comprises a feedback and control circuit, such as a pulse-width-modulation controller, for controlling the driving circuit according to the current values of the lamp set.